1. Field of the Invention
The present invention relates to a transfer fixing device and an image forming apparatus incorporating the same, and more particularly, to a transfer fixing device that simultaneously transfers and fixes a toner image into place on a recording medium, and an electrophotographic image forming apparatus, such as a photocopier, printer, facsimile, or multifunctional machine having several of those image imaging functions, incorporating such a transfer fixing device.
2. Discussion of the Background
In electrophotographic image forming apparatuses, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by electrostatically by attracting toner particles to a photoconductive surface. After image formation, the powder toner image is transferred to a recording medium such as a sheet of paper, followed by a fixing process that permanently fixes the toner image in place by melting and settling toner with heat and pressure.
Various types of electrophotographic systems are known in the art, some of which employ a simultaneous transfer fixing or “trans-fixing” process that simultaneously transfers and fixes a toner image into place on a recording sheet. Typically, a trans-fixing device uses a trans-fixing belt or cylinder with a heated, moving surface on which a powder toner image travels from where it is formed or intermediately transferred, and a pressure member pressed against the trans-fixing member to define a trans-fixing nip where the toner image meets a recording sheet traveling through a sheet conveyance path. Upon entering the trans-fixing nip, the toner image melts and settles onto the recording sheet under heat and pressure, and subsequently cools and solidifies to obtain a finalized image.
One advantage of using a trans-fixing process in electrophotographic printing is that it accommodates use of smooth recording media as well as rough recording media with surface roughness or vertical irregularities on the order of several micrometers.
In an electrostatic transfer device separate from a thermal fixing process which transfers toner from a transfer member to a recording sheet under an electrostatic bias field, printing on rough paper results in the transfer member failing to conform to the microscopic surface irregularities of the recording sheet. This creates minute gaps between the transfer member and paper surface, which affect proper formation of the electrostatic bias field and hence proper transfer of toner to desired portions of the recording sheet, causing loss of detail and a grainy, fuzzy appearance of a resulting print.
By contrast, a trans-fixing device can render toner particles into a viscoelastic mass on a trans-fixing member with heat during simultaneous transfer and fixing, which readily transfers in an image configuration onto a recording sheet even across minute gaps present within the trans-fixing nip. Such thermally-assisted transfer yields an extremely clear, sharp image regardless of whether the recording sheet in use is smooth or rough.
Another advantage of using trans-fixing process is that it allows a high degree of flexibility in designing the sheet conveyance path along which a recording sheet is fed through the electrophotographic system.
Conventionally, designing a sheet conveyance path extending from transfer to fixing units involves a concern that an unfixed, powder toner image smears or rubs off when the recording sheet bearing it encounters surrounding structures between the transfer and fixing processes. This places constraints on the design of equipment for guiding recording sheets in the sheet conveyance path, making it difficult to provide an electrophotographic system with effective sheet conveyance performance.
Such concern is obviated by using simultaneous transfer and fixing in place of separate transfer and fixing processes. That is, with a trans-fixing unit that performs both transfer and fixing within a trans-fixing nip, an image forming apparatus can convey a recording sheet therethrough without exposing an unfixed image to neighboring structures. This allows a wide range of designs to become available for effective sheet conveyance, leading to increased efficiency and productivity of an image forming apparatus incorporating the trans-fixing unit.
Owing to such advantages over separate transfer and fixing, the trans-fixing method has found application in high-quality color printers accommodating a wide range of recording media with consistent, and consistently superior, imaging quality. However, with growing demands for energy-efficient products with even greater performance, severe requirements are currently imposed on those trans-fixing printers in terms of thermal efficiency and compatibility with extremely rough recording media.
As mentioned above, a typical trans-fixing device fuses toner on a heated surface of a trans-fixing member traveling from where a toner image is formed or intermediately transferred from another surface to where the toner image is transferred to a recording sheet (i.e., the trans-fixing nip). Heating the trans-fixing member can thus result in a significant amount of heat absorbed and lost over the length of trans-fixing member, particularly where the trans-fixing surface has a substantial thickness for obtaining durability, or, as in the case of tandem color printers, a substantial length for traveling across multiple imaging units. Such heat loss becomes even more pronounced where the trans-fixing member is cooled downstream of the trans-fixing nip to prevent thermally-induced damage to components arranged along the endless travel path, resulting in large amounts of energy lost in the repeated thermal cycle.
Moreover, although simultaneous transfer and fixing operates well with rough recording media compared to electrostatic transfer separate from thermal fixing, it can encounter a decrease in transfer efficiency when the recording medium in use has an extremely rough surface with irregularities on the order of 30 micrometers or more. This results in inconsistent image resolution and definition, and a limited range of recording media accommodated in an image forming apparatus incorporating the trans-fixing process.
Both of these current requirements should be met to provide a successful trans-fixing device for application to today's high quality electrophotographic systems. The thermal energy issue can be alleviated by providing a supplementary heater that heats a recording medium upstream of the trans-fixing nip to minimize an amount of heat applied to a trans-fixing member for simultaneous transfer and fixing. However, such alleviation is not fully exploited in the art, and no effective solution has been proposed to address the requirement of compatibility with extremely rough recording media.